Physical stability of colloidal dispersions depends on the balance of the following forces:
Physical stability
of colloids
Physical
stability of colloidal dispersions depends on the balance of the following
forces:
1.
Electrical forces of repulsion between dispersed-phase
particles
2.
Forces of attraction between dispersed-phase particles
3.
Forces of attraction between the dispersed phase and the
dispersion medium
Accordingly,
colloidal dispersions can be stabilized by the following:
1. Modulating the electric charge on the dispersed
particles. The pres-ence and magnitude, or the absence, of a charge on a
colloidal particle are important determinants of the stability of colloidal
systems. This can be done through ion adsorption, dissociation of ionizable
func-tional groups, and ion dissolution. In addition, ionized species added to
the aqueous solution, such as salt, can influence the overall zeta potential on
the surface of the dispersed phase.
2. Surface coating of the particles to minimize adherence on
collisions. This effect is significant for hydrophilic colloids. Thus, addition
of soluble hydrophilic polymers to colloidal dispersions can entangle
dispersed-phase particles, minimizing the speed and impact of inter-particle
collisions.
The
stabilizations strategy depends on the type of colloid and the specific
properties of a colloidal system.
Stabilization of
hydrophilic colloids
Hydrophilic
and association colloids are thermodynamically stable and exist in a true
solution, so that the system constitutes a single phase and is visually clear.
When negatively and positively charged hydrophilic colloids are mixed, the
particles may separate from the dispersion to form a layer rich in the
colloidal aggregates. The colloid-rich layer is known as a coac-ervate, and the phenomenon by which macromolecular solutions
separate into two liquid layers is
referred to coacervation. For
example, when the solutions of gelatin and acacia are mixed in a certain
proportion, coacer-vation results. Gelatin at a pH below 4.7 (its isoelectric
point) is positively charged, whereas acacia carries a negative charge that is
relatively unaf-fected by pH in the acid range. The viscosity of the outer layer
is markedly decreased below that of the coacervate, which is considered as incompat-ibility. Coacervation need not
involve the interaction of charged particles. Coacervation of gelatin may also be brought about by the addition
of alco-hol, sodium sulfate, or a macromolecular substance such as starch.
In
colloidal dispersions, frequent interparticle collisions due to Brownian
movement can destabilize the system. Thus, increase in temperature often
compromises the physical stability of these systems.
Stabilization of
hydrophobic colloids
In
contrast to hydrophilic colloids, lyophobic or hydrophobic colloids are
thermodynamically unstable but can be stabilized by imparting electric charge
on the dispersed particles, which can prevent aggregation by increas-ing the
repulsion between like particles. Addition of a small amount of elec-trolyte to
a hydrophobic colloid tends to stabilize the system by imparting a charge to
the particles. Addition of excess amount of electrolyte may result in the
accumulation of opposite ions and reduce the ζ potential below its critical value,
leading to destabilization. The critical potential for finely dispersed oil
droplets in water is about 40 mV. This high value indicates relative
instability and the need for significant electrostatic charge repul-sion for
stabilization. In comparison, the critical ζ potential of colloidal gold is
nearly zero, which suggests that the particles require only a minute charge for
stabilization.
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